1. Technical Field
The present disclosure relates to a battery voltage control device and a battery voltage control method.
2. Related Art
Japanese Unexamined Patent Application Publication (JP-A) No. 8-308025 describes that, when an inverter DC current is lower than or equal to a prescribed discharge current during acceleration of a vehicle, a converter is controlled to turn off, and when the inverter DC current exceeds the defined discharge current, the converter is controlled to step up a voltage so that a second electric condenser supplies the exceeding amount of the current. Further, JP-A No. 8-308025 discloses that, when the inverter DC current is lower than or equal to the prescribed discharge current during deceleration, the converter is controlled to turn off, and when it exceeds the prescribed discharge current, the converter is controlled to step down the voltage so that the exceeding amount of the current flows to the second electric condenser.
In addition, vehicles that use electric motors such as electric automobiles, hybrid automobiles, and fuel cell automobiles have recently garnered attention as eco-friendly automobiles. In such a vehicle, it is assumed that a DC current from a power supply is stepped up by a step-up converter and the stepped up voltage is converted into an AC voltage to drive a motor. For example, Japanese Patent No. 4802849 describes that a step-up voltage is changed according to a state of an eco-switch being on or off.
Moreover, Japanese Unexamined Patent Application Publication (JP-A) No. 2009-292383 describes execution of step-up control of a hybrid automobile and the like in which a voltage to be supplied to an electric motor is stepped up in advance in preparation for a request for a driving force increase. In addition, JP-A No. 2009-292383 describes that it is determined whether or not a subject vehicle will pass a preceding vehicle based on information from a blinker sensor.
In step-up control by a step-up converter, however, after step-up is started, a delay occurs until a voltage reaches a target step-up voltage. In the technology described in JP-A No. 8-308025, the converter is controlled to step up the voltage when the inverter DC current exceeds the prescribed discharge current, and thus there are problems that it is not possible to instantly obtain a maximum output due to such a response delay of the converter, a feeling of lagging acceleration occurs, and start acceleration performance deteriorates.
Furthermore, when the step-up converter performs step-up control, a loss of the step-up converter increases as a step-up voltage becomes higher. In the technology described in JP-A No. 8-308025, when a vehicle stops from a travel state, the converter is set to turn off when the inverter DC current is lower than or equal to the prescribed discharge current, and thus there are problems that it is not possible to immediately stop the converter, and reduction of a loss of the step-up converter becomes late. In addition, if a step-up voltage is decided by adding a margin to a target voltage in order to cope with the aforementioned response delay, there is a problem that a loss of the step-up converter further increases.
Moreover, in the technology described in Japanese Patent No. 4802849, the states of the eco-switch being on and off are considered during step-up, but control according to a travel mode that relates to an output characteristic of a driving motor (for example, a sport mode, an intelligent mode, or the like) is not considered at all. For this reason, problems that it is not possible to optimally control a step-up voltage according to a travel mode, an increase in a loss caused by excessive step-up, deterioration in acceleration performance caused by insufficient step-up, and the like are assumed to occur.
Furthermore, when the step-up converter performs step-up control, a loss of the step-up converter increases as a step-up voltage becomes higher. In the technology described in JP-A No. 2009-292383, the supply voltage to the electric motor is stepped up in advance in preparation for the request for a driving force increase, but if step-up of the supply voltage increases more than necessary, an increase of a loss is problematic. Thus, in the technology described in JP-A No. 2009-292383, it is not possible to precisely perform step-up control and it is difficult to optimally control step-up of a voltage with a loss suppressed to its minimum.